The present invention relates to optical attenuators. EP-A-557542 discloses an optical attenuator for attenuating the intensity of a light beam emerging from an optical fiber. The attenuator comprises a wedge-shaped attenuator disk made of an absorbing material and a transparent disk that is also wedge-shaped and fixed to the absorbing disk. For adjusting different attenuations, the attenuator disk together with the transparent disk are rotated around an axis, whereby the thickness of the absorbing material through which the beam passes and thus the attenuation of the beam can be varied continuously. The problem with this solution is that the high local power consumption in the absorbing disk, which leads to heating and eventually destruction on the absorbing disk. Moreover, the attenuation shows a dependency on the wavelength.
U.S. Pat. No. 5,015,057 discloses a polarization insensitive optical attenuator that uses a polymer-dispersed liquid crystal (PDLC) film to provide attenuation control over a wide range of attenuation values. This solution, however, is only applicable in a small temperature range, which renders necessary use of a temperature regulation. Additionally, such PDLC attenuators show a hysteretic behavior, which complicates the control of the attenuation. Moreover, it has been shown that polymers generally do not exhibit sufficient long-term stability due to aging effects. Finally, such PDLC attenuators are not applicable for attenuating high power optical beams.
An adjustable attenuator for optical transmission systems is disclosed in U.S. Pat. No. 5,087,122 and comprises a blocker having a vane-like structure which is gradually rotated into a collimated beam to cause attenuation.
EP-Axe2x80x94341919 discloses a light attenuator comprised of a thermoplastic resin layer in which finely divided particles having light-absorbing or light-scattering property are uniformly incorporated.
It is an object of the invention to provide an improved optical attenuator for fiber-optical applications. The object is solved by the independent claims. Preferred embodiments are shown by the dependent claims.
According to invention, a substantially parallel optical input beam is directed onto a scattering element of an attenuator. The scattering element scatters the input beam into a range of scattering directions. A beam collecting device, such as a collecting lens, of the attenuator is arranged in the range of scattering directions, and collects a portion of the scattered light as an output beam into an output fiber. The attenuation of the output beam with respect to the input beam is thus dependent on the portion of the collected light relative to the input beam.
The invention thus allows providing optical attenuators for high-power applications. Further more, the wavelength dependency for the provided attenuation can be minimized by selecting the particle size and distribution. Finally, there is virtually no limitation for long-term stability, since heating and thus aging effects can be effectively avoided and the attenuation itself does virtually not depend on the temperature of the scattering material.
In a preferred embodiment, the attenuator is provided with a shielding casing, at least in the range of scattering directions, in order to protect persons and equipment from light beams exciting the attenuator and to shield an output fiber from ambient light.
The scattering element is provided with a varying scattering angle distribution in order to control the attenuation. This can be done e.g. in that a volume scattering element is provided having a wedge-shape as disclosed e.g. by the aforementioned EP-A-55742. This allows controlling the attenuation by means of the effective thickness of the scattering element in the optical path. The scattering element can be provided having disk shape or a longitudinal shape. In the former case, the attenuation can be varied be rotating the disk. In the latter case, the attenuation can be varied be moving the longitudinal scattering element e.g. perpendicular to the input beam.
In another embodiment, the varying scattering angle distribution of the scattering element is provided by a gradient of the density of the scattering particles or by a varying surface scattering angle distribution created for example by chemical etching, photofabrication or sandblasting.